Various methods exist for forming a workpiece. In hydro-forming for example, a tubular workpiece is filled by a liquid, usually water, and is subsequently sealed. By increasing the liquid pressure the workpiece is expanded and gradually adapts to the contours of the forming tool surrounding the workpiece.
An explosion forming method of the above-mentioned kind not using a liquid is described in EP 592 068. To manufacture a camshaft a mold is fit with the prefabricated cams and is closed after an internally hollow camshaft shaft is passed through the openings of the individual cams. The ends of the closed mold are sealed by sealing elements and a spark plug projecting into the camshaft shaft is screwed-in. After the shaft has been filled by a combustible gas, it is ignited by means of the spark plug. Caused by the sudden rise of the gas pressure in the interior of the shaft, this shaft is expanded and pressed into the openings of the individual cams. The cams are therefore axially connected with the camshaft shaft in a manner fixed for co-rotation.
This document discusses the conventional disadvantages of explosion forming. Due to the shock wave following the detonation, the workpiece tends to local formation of tension and cracks. The high peak pressure generated causes an inconstant flow of material, which can lead to differences in wall thickness. In EP 592 068 it is proposed to cure this in that the forming pressure is carried out by a deflagration instead of an explosion (detonation). A detonation is based on a chemical reaction of the explosive(s) and propagates by shock wave induced combustion. A superposition of the pressure wave with the volume expansion takes place, which leads to the higher speed and multiple pressure compared to a deflagration. Contrary thereto, a deflagration is a fast combustion process which propagates by the heating-up of the unused mixture. The pressure reached by the gas expansion is approximately 10 bar and the speed is significantly lower than the speed of sound.
This approach failed to be implemented in practice due to the fact that on the one hand a deflagration is less process safe and, on the other hand, the speed and the forming pressure are lower compared to a detonation.